There are known attempts to solve this problem by adding organic solvents (G. Carrea, S. Riva, Angew. Chem. 2000, 112, 2312; J. M. S. Cabral, M. R. Aires-Barros, H. Pinheiro, D. M. P. Prazeres, J. Biotechnol. 1997, 59, 133; M. N. Gupta, Eur. J. Biochem, 1992, 203, 25), by adding salts (A. M. Blinkorsky, Y. L. Khmelnitzky, J. S. Dordick, J. Am. Chem. Soc. 1999, 116, 2697) or by carrying out the reaction in microemulsions (B. Orlich, R. Schomäcker 1999, 65, 357-362). Frequently however, the improvements achieved thereby are not significant and do not justify the additional expenditure, or the enzyme stability decreases severely under these conditions (G. Carrea, S. Riva, Angew. Chem. 2000, 112, 2312). At low temperatures (<100° C.) ionic liquids are melting salts which represent a new class of solvents having a non-molecular ionic character. Although the first representatives have been known since 1914, ionic liquids have only been investigated intensively as solvents for chemical conversions in the last 15 years. Ionic liquids have no measurable vapour pressure. This is a major advantage from the process engineering point of view because in this way, the distillative separation of a reaction mixture is possible as an effective method for product separation. The known problems caused by azeotrope formation between solvents and products do not occur, Ionic liquids are temperature-stable up to above 200° C. By means of a suitable choice of cation and anion, it is possible to gradually adjust the polarity and thereby tune the solubility properties. The range goes from water-miscible ionic liquids through water-immiscible ionic liquids as far as those which themselves form two phases with organic solvents. The skilful utilisation of the extraordinary solubility properties is the key to the successful use of ionic liquids as a new class of solvents.
Ionic liquids have already been successfully used as new types of media in two-phase catalysis or as the medium for liquid-liquid extraction). (P. Wasserscheid, W. Keim, Angew. Chem. 2000, 112, 3926).
According to the invention a substantial increase in the yield and selectivity was surprisingly established during the conversion of a wide range of educts with different enzymes in the presence of ionic liquids, which represents a significant improvement compared with the prior art. No adverse effects of the ionic liquid on the enzyme stability were established and in individual cases, even a stabilising effect was found.
This is unexpected and surprising bearing in mind the ionic nature of the ionic liquids and the strong interactions thereby possible between the ionic liquids and the enzyme with its likewise charged groups.
It was also found that ionic liquids can be used as co-solvents to improve the solubility of educts and products.